A typical ethylene polymerization catalyst is prepared by forming a precursor from a magnesium compound, a titanium compound, and an electron donor compound; diluting the precursor with an inert carrier material; and activating the precursor by introducing an organoaluminum compound. The process is described in U.S. Pat. Nos. 4,302,565; 4,302,566; and 4,303,771, incorporated by reference herein. The magnesium and titanium compounds are dissolved in the electron donor compound (solvent) at a temperature ranging from ambient to below the boiling point of the electron donor. The order of addition to the electron donor compound is not important to the result, i.e., one or the other of the magnesium and titanium compounds can be added first or they can be added together. The dissolution in the electron donor compound can be enhanced by slurrying or refluxing. After the magnesium and titanium compounds are dissolved, the resulting product is isolated by crystallization or precipitation with a hydrocarbon such as hexane, isopentane, or benzene. The crystallized or precipitated product is dried and recovered as fine, free-flowing particles. The magnesium/titanium based composition is then mixed with, or impregnated into, an inert carrier material. The carrier is generally a solid, particulate, porous material such as silica.
In order for the magnesium/titanium based composition to be useful as a polymerization catalyst, it must be activated with a compound capable of transforming the magnesium/titanium atoms to a state which will effect the desired polymerization reaction. Activation is accomplished by the addition of an organoaluminum compound. Partial activation, if desired, is effected outside of the polymerization reactor by introducing the catalyst composition and the organoaluminum into a solvent. Complete activation is then carried out in the reactor as described in U.S. Pat. No. 4,383,095, incorporated by reference herein.
While the magnesium/titanium based catalyst compositions have proved to be satisfactory ethylene polymerization catalysts, there is a continuing effort to improve on the catalysis aspect of ethylene polymerization and, more particularly, to improve the technique for catalyst activation.